Components of steam turbines, gas turbines, gas turbine engines, and jet engines experience a range of operating conditions along their axial lengths. Not only do the different operating conditions complicate the selection of a suitable casting material and manufacturing, but also the material and manufacturing of the stationary components of such turbines are impacted. For example, a material optimized to satisfy one operating condition may not be optimal for meeting another operating condition. For instance, the inlet and exhaust areas of a steam turbine casting have different material property requirements compared to requirements of a gas turbine. For example, steam turbine casings in general are pressurized chambers at high temperatures and are hence are creep limited. On the other hand, gas turbine casings are typically exposed to frequent thermal cycling and could be fatigue limiting. These properties, which are sometimes conflicting, are tailored with a suitable mix of heat treatment cycles to achieve an optimum mixture of strength, toughness, creep and fatigue properties, depending on application.
For casings and other casting components, the steam turbine industry currently favors CrMoV low alloy steels for temperatures below 1050° F. As higher inlet temperatures are sought, for example up to about 1060° F. (about 575° C.), to increase steam turbine efficiencies, chromium steel alloys having about 9 to 14 weight percent chromium with varying levels of Mo, V, W, Nb, B are typically be used to meet the higher temperature conditions in the HP stage of the steam turbine. While capable of operating at temperatures exceeding 565° C. within the HP stage of a steam turbine, casting components produced from these alloys incur higher costs and additional measures are often required to address thermal expansion mismatches with alloys used in the casting components of cooler stages.
Not only are such high-Cr steel alloys expensive to produce, but also they are not particularly well suited for the casting processes utilized to form various stationary components of such turbines (e.g., shell, valve, diaphragm, packing head, or packing ring). Currently, various stationary components of such turbines are typically made with CrMoV steel alloys (for components exposed to temperatures up to 1050° F.) and 9-12% Cr steel alloys (for applications that require either higher temperature or stress). In high temperature applications, the cost of the 9-12% Cr steel alloys—mainly due to the relatively high amounts of Cr present—can significantly impact the design, component selection, and final cost of the turbine.